Impact of inflation pressure retention on tire aging and durability.

The innerliner is a thin layer of rubber laminated to the inside of a tubeless tire whose primary function is vapor retention. It is formulated to also provide flex fatigue resistance, aging resistance and adhesion to the ply coat compound (refs. 1 and 2). It has been shown that of the elastomers studied, butyl rubber has the lowest permeability coefficient (ref. 3). Butyl rubber (IIR) is the copolymer of isobutylene and about 2% of isoprene (refs. 4-8). The impermeability of polyisobutylene is thought to be the result of the close packing of the geminaldimethyl side groups along the polymer backbone, which results in slow movement of the chains (ref. 9). As an example, three repeat units of cis-polyisoprene totaling 15 carbon atoms occupy more space than do four repeat units of polyisobutylene that contain 16 carbon atoms. This is a result of the [sp.sup.3] hybridization of each carbon atom in polyisobutylene affording a tetrahedral arrangement of the backbone, compared to the planar arrangement of the [sp.sup.2] hybridized carbon atoms in the C=C double bond of each segment in polyisoprene.

The development of halogenated butyl rubbers greatly extended the usefulness of butyl rubbers by providing faster curing rates, which enabled co-vulcanization with general-purpose tire rubbers used in carcass coat stocks such as natural rubber, styrene-butadiene rubber and butadiene rubber, without affecting the desirable impermeability and fatigue properties. This permitted development of tubeless tires in which the innerliner compound is chemically bonded to the carcass ply.

Bromobutyl rubber is used extensively in innerliner formulations for several reasons, including (refs. 10-22):

* Superior adhesion and balance of properties;

* increasing use of ultra-low profile, speed-rated tires having higher surface area to air volume ratios;

* requirement for lighter tires to reduce rolling resistance and improve vehicle fuel economy;

* use of high-pressure space-saver spare tires requiring a more impermeable innerliner;

* better flex-cracking resistance after aging; and

* bromobutyl innefliners, at half the gauge, are cheaper in material costs

Previously, we have presented results (refs. 23-25) of the effect that the bromobutyl rubber content in the innerliner formulation has upon tire performance such as inflation pressure retention (IPR), intracarcass pressure (ICP) and durability as measured on laboratory road wheels. Innerliners with 100 phr bromobutyl rubber and 80/20 and 60/40 blends of BIIR/ NR were studied in P205/60 H15 and P205/60 SR15 radial passenger tires. Tires with a 100 phr bromobutyl innerliner had the lowest IPR (figure 1), lowest ICP (figure 2) and highest laboratory roadwheel durability values (figures 3 and 4). Now we present results showing the effect that tire inflation pressure retention has upon the tire aging process of the P205/60 SR15 radial passenger tires.

Experimental

The bromobutyl rubber used was BIIR 2222 (ExxonMobil Chemical) and the natural rubber was SMR 20. All other ingredients were commercial materials. Model innerliner formulations affording different IPR values were used (table 1). Compounds were factory mixed using a two-step sequence in internal mixers having tangential rotors. Masterbatch mixing was completed using a GK400 mixer followed by sheeting out on an extruder with roller die. Finalization was completed in a GK160 mixer and stocks sheeted out on a two-roll mill. A cold-feed pin extruder was used to profile each innerliner compound (refs. 23-25).

P205/60 SR15 tires were built with the three model innerliner compounds using automatic building machines, and tires were cured as usual. Cured tire innerliner thicknesses averaged 1.0 mm. Inflation pressure retention loss rates were measured in accordance to ASTM F-1112 procedures and are reported as the percent pressure loss per month. Intracarcass pressure was measured for tires on standard rims inflated to 240 kPa. Typically, five calibrated gauges with hypodermic needles were inserted, with the needle tip set on the cords. Readings were taken daily until the pressure equilibrated, normally two months, and are reported as the average of at least two tires.

A tread separation test was run for tires inflated to 240 kPa using a dry 50/50 [O.sub.2]/[N.sub.2] mixture. Tires were run against a 28.5 cm diameter laboratory roadwheel at 80 km/hr, at 21[degrees]C. A load was applied to give a deflection of 30%. The tire is run for one hour at 50% load followed by one hour at 100% load. The inflation pressure was then recorded while the tire was hot and was adjusted daily to this level until tire failure occurred. Results are reported as hours run until failure.

The Federal Motor Vehicle Safety Standards (FMVSS) 139 test procedures were performed using a 1.7 m diameter laboratory roadwheel of an All Well tire testing machine, Model AW-TT-2A-M4, running at 38[degrees]C in our laboratory in Bangalore, India. Briefly, the FMVSS 139 endurance procedure was to run the air-inflated (180 kPa) tire at 120 km/h. for four hours at 85% load, followed by six hours at 90% load, and then followed by 24 hours at 100% load. Tires were inspected visually for any failures. If none were observed, testing was continued by following the FMVSS 139 low inflation test procedure, which is to run for 90 minutes at 100% load with 120 kPa air inflation.

The Akron Rubber Development Laboratory (Akron, OH) was employed to analyze new and oven aged/roadwheel tested tires. Shearography (ref. 26) was performed using an ITT Compact (SDS Systemtechnik GmbH). Tires were dissected and the natural rubber skim coat and shoulder wedge compounds were analyzed for changes in chemical and physical properties. Crosslink density of the skim coats was determined by solvent swelling (ref. 26). Fixed oxygen contents of the skim coats were determined by elemental analysis using a LECO CHNS-932. Tensile properties (ASTM D412) were determined for the NR wedge compounds. One-inch wide strips were used to determine the peel adhesion strengths (ASTM D413) for the wedge and skim composites. Modulus profiles (refs. 26-30) were generated in the shoulder areas of the tire.

SAS JPM 4.0 software was used to statistically analyze all data.

Results and discussion

Physical and aged physical properties of the innerliner compounds have been reported (ref. 24). Aged properties were adversely changed when bromobutyl rubber was replaced with natural rubber in the innerliner formulation. Air permeability showed a quantitative, linear ([R.sup.2] = 0.99) decrease with increasing bromobutyl rubber content in the innerliner compound.

New tires

Tire inflation pressure retention (IPR; see figure 1, [R.sup.2] > 0.99) and intracarcass pressure (ICP; see figure 2, [R.sup.2] > 0.96) for both the P205/60 H15 and P205/60 S15 sets of radial passenger tires decreased linearly with increasing bromobutyl rubber content in the innerliner compound. The tire having a 100 phr bromobutyl rubber innerliner has the desirably lowest Tire IPR and tire ICP values, regardless of the speed rating.

Two laboratory roadwheel durability tests were performed, including tread separation (28.5 cm diameter laboratory roadwheel), and FMVSS 139 endurance (1.7 m diameter laboratory roadwheel), but testing until tire failure. All tire failures were the result of belt area separations such as belt-to-belt and/or belt-to-cap ply. Tires run on the small (28.5 cm) laboratory roadwheel at 80 km/h. until failure showed a high statistical correlation to the tire inflation pressure retention loss rate values for both the H-rated ([R.sup.2] = 0.95) and S-rated ([R.sup.2] > 0.94) tire sets (figure 3).

P205/60 HR15 tires were run on the 1.7-m test wheel according to FMVSS 139 Endurance at 120 km/h. until the 34-hour test period had elapsed. Tires were inspected, had no visible flaws, and then were run until failure. A good correlation of hours to failure versus tire 1PR loss rates ([R.sup.2] = 0.88) was obtained (figure 4). For each laboratory roadwheel durability test, the tire having the lowest inflation pressure retention loss values performed best, as determined by the longest number of hours run before failing by a belt area separation.

P205/60 SR15 tires were run on the 1.7 m test wheel according to three FMVSS 139 standards, including high speed, endurance and low inflation. All tires were inspected after the respective testing time period, had no visible flaws, and then were run until failure. No correlations were obtained (figure 5).

Characterization of roadwheel-tested tires

Tires with varying polymer ratios in the innerliners that had run until failure on the tread separation tester (28.5 cm wheel) were dissected and specific components analyzed. They were the P205/60 HR15 tires with IPR values of 1.48, 2.05 and 2.75. Since all components other then the innerliner polymer ratio were identical, one new tire was analyzed to serve as a control to obtain original values. Chemical (crosslink density and fixed oxygen content) and physical (tensile, peel strength and modulus profiles) properties were determined.

The crosslink density values of the skim coat or shoulder wedge and the 100% modulus values of the wedge had no significant correlation to tire IPR values. Elongation to break values of the wedge showed a qualitative correlation to tire IPR ([R.sup.2] = 0.77). Belt-to-belt peel strength values of the skim composites showed good correlations to tire IPR values ([R.sup.2] = 0.84). Similar good correlations were also obtained for the peel strength of the wedge composite to tire IPR ([R.sup.2]--0.89).

Fixed oxygen contents (covalently bonded oxygen) of the natural rubber skim coat and shoulder wedge compounds showed good correlations to tire IPR ([R.sup.2] = 0.8). However, since tires failed at different times on the roadwheel, data were normalized for these differences in the number of hours run until failure on the tread separation tester. As a result of this mathematical normalization, quantitative statistical correlations were obtained for the fixed oxygen contents of the skim coat ([R.sup.2] > 0.999) and the shoulder wedge compounds ([R.sup.2] = 0.999) to tire inflation pressure retention loss rates for both the P205/60 HR15 and P205/60 SR15 tire sets. Figure 6 illustrates results obtained for the skim coat compound of the P205/60 HR15 tire set with and without normalizing for tire failure time differences. These results indicate that even though the high performance H-rated tires perform better on laboratory roadwheel tests when measured as hours to failure, their tire components still oxidize at the same rate as the S-rated tires. As a follow-up experiment suggested by these results, tires were oven aged to an equal extent and then were roadwheel tested to the same time periods in order to obtain a direct comparison of the effects of oxidation on the natural rubber skim coat and shoulder wedge components without the need for data normalization.

Characterization of oven-aged roadwheel-tested tires

New P205/60 SR15 tires were inflated with dry air to 240 kPa (35 psi), placed vertically on a rack and aged in an air-circulating walk-in oven for 28 days at 70[degrees]C. Tire inflation pressures were not monitored, nor were they re-inflated during this time period. Tires were dismounted and shipped to our laboratory in Bangalore, India, where they were tested according to the FMVSS 139 endurance test standard. After completing the 34-hour test, tires were inspected and had no visible flaws. The 90-minute FMVSS 139 low inflation test was completed on these tires. They were inspected again with no flaws visible. Tires were dismounted again and shipped for follow-up component characterizations.

Akron Rubber Development Laboratory (ARDL) performed shearography analysis (ref. 26) for the three oven-aged tires and a new tire prior to dissection. The new tire contained a 100 phr bromobutyl rubber innerliner that served as a control for all chemical and physical characterizations. The crosslink density values and belt-to-belt peel strength values of the natural rubber skim coat or shoulder wedge compounds (figure 7), and the 100% modulus and elongation to break values of the wedge did not have any statistically significant correlations.

[FIGURE 7 OMITTED]

Fixed oxygen contents of the natural rubber skim coat and shoulder wedge compounds showed quantitative statistical correlations to tire IPR ([R.sup.2] > 0.99) values without the need for normalization of data since in these experiments they had been identically treated in both the oven aging and roadwheel testing procedures. Thus, even though these three tires have different innerliner polymer ratios affording different tire IPR values, a statistically very significant correlation was obtained (figure 8).

Samples were obtained in the shoulder region for the three oven-aged and roadwheel-tested tires and the new tire. Inspection of the micrographs showed varying degrees of cracking (figure 9). Modulus profiles were obtained from these shoulder region samples and generally showed increased modulus values for the individual components of the tire with increasing inflation pressure retention loss values (figure 10).

[FIGURES 9-10 OMITTED]

Shearography measurements showed that there was no cracking in the new tire. The tire with the lowest inflation pressure loss value (IPR = 1.45% air loss/month) had 642 [mm.sup.2] of cracking at the belt edges upon integration of all observed shoulder cracking. The fire with an IPR = 2.0 had more than 12,000 [mm.sup.2] of total cracking at the belt edges, and the fire with the highest IPR value (IPR = 2.65) had more than 25,000 [mm.sup.2] of total cracking at the belt edges. A very significant statistical quantitative correlation is obtained between shearography total crack areas and the tire IPR values with [R.sup.2] > 0.99 (figure 11).

A statistically significant quantitative correlation can also be obtained between the dependent variables. The total crack areas at the belt edges measured by shearography and the fixed oxygen contents of the natural rubber skim coat (figure 12) and shoulder wedge components have [R.sup.2] > 0.99. This correlation indicates that the cracking probably is the direct result of oxidation of the natural rubber compounds. A statistically significant quantitative correlation is also obtained between the total crack areas at the belt edges measured by shearography and the modulus values of the skim coat 1 component obtained from the profiling measurements ([R.sup.2] > 0.99) (figure 13). This latter correlation can be attributed to an increasing modulus value with increasing oxidation and/or with increasing total crosslink values. However, since the increase in total crosslinks is similar for all of the three tires (figure 7), the observed increase in modulus leading to cracking can be reasonably attributed to oxidation effects of the skim coat.

[FIGURES 12-13 OMITTED]

Thus, a possible mechanism consistent with these direct measurements is that the skim coat compound oxidizes, resulting in increasing modulus which affords cracking at the belt edges when sufficient stress (i.e. % load on road wheel tests) is applied to the tire. The tire with the lowest IPR values affords the lowest skim coat oxidation and modulus values, and significantly lower total belt edge crack areas.

Summary

The integrity of the tire is improved significantly by using a 100 phr bromobutyl rubber formulation in the innerliner, since it minimizes tire inflation pressure retention (IPR) loss rate values and minimizes the development of tire intracarcass pressure (ICP) values. Statistically significant correlations were obtained for both H-rated and S-rated radial passenger tires.

The durability of new tires, as determined by measuring the hours to failure by a belt area separation on a laboratory roadwheel, is maximized by using innerliners that afford the lowest inflation pressure retention loss rate values. Statistical analysis of durability hours to failure to tire IPR values showed quantitative correlations ([R.sup.2] > 0.94) based upon tread separation testing of the H-rated and S-rated tires. Good correlations ([R.sup.2] = 0.88) to the FMVSS 139 Endurance test (modified by running until tire failure) were obtained for the H-rated tires.

Good statistical correlations were obtained for the roadwheel tested tires run to failure on the tread separation tester for the belt-to-belt peel strengths and the fixed oxygen contents of the natural rubber skim coat and shoulder wedge composites versus the inflation pressure retention loss rates of the tire. Quantitative ([R.sup.2] > 0.99) correlations to tire IPR were obtained when values were normalized according to the hours run until failure. Modulus profiling studies show that the tire with the lowest tire IPR values best protected the other internal rubber components of the tire from age hardening.

For oven-aged and roadwheel tested P205/60 SR15 tires, statistically significant quantitative correlations ([R.sup.2] > 0.99) were obtained between tire IPR values and (1) fixed oxygen values directly measured for the natural rubber skim coat and shoulder wedge components, (2) modulus values of the skim coat 1 (belt nearest the tire innerliner) directly measured by profiling experiments, and (3) total cracking in the belt-edge area directly measured by shearography. Statistically significant quantitative correlations ([R.sup.2] > 0.99) were also obtained between dependent variables. Namely, the shearography total crack area at the belt edges correlates to the (1) fixed oxygen contents of the natural rubber skim coat and shoulder wedge components, (2) modulus values of the skim coat 1 (belt nearest the tire innerliner) directly measured by profiling experiments.

A possible mechanism consistent with these direct measurements is that the skim coat compound oxidizes, resulting in increasing modulus, which affords cracking at the belt edges when sufficient stress is applied to the tire. The tire with the lowest IPR values affords the lowest skim coat oxidation and modulus values, and significantly lower total belt edge crack areas.

This article is based on a paper presented at a meeting of the Rubber Division, ACS (www.rubber.org).

References

(1) R.S. Bhakuni, S.K. Mowdood, W.H. Waddell, I.S. Rai and D.L. Knight, "Tires" in Encyclopedia of Polymer Science and Engineering, 2nd Ed, 16, p. 834, John Wiley & Sons Inc, New York, 1989.

(2.) W.H. Waddell, R.S. Bhakuni, W. W. Barbin and P.H. Sandstrom, "Pneumatic tire compounding" in The Vanderbilt Rubber Handbook, R.F. Ohm, ed., p. 596, R.T. Vanderbilt and Co, 1990.

(3.) G.J. van Amerongen, "Diffusion in elastomers," Rubber Chem. Technol., Rubber Reviews, 37, 1,065 (1964).

(4.) E.N. Kresge and H-C. Wang, "Butyl rubber" in Kirk-Othmer Encyclopedia of Chemical Technology, 4th Ed, M. Howe-Grant, ed., 8, 934, John Wiley & Sons, New York, 1993.

(5.) J.E. Rogers and W.H. Waddell, Rubber World, 219, 24, February 1999.

(6.) G.E. Jones, D.S. Tracey and A.L. Tisler, "Butyl rubber" in

Walter H. Waddell, R. Christopher Napier, Dirk F. Rouckhout and Donald S. Tracey, ExxonMobil Chemical www.butylrubber.com

Table 1--innerliner compound formulations

Ingredient                          1      2      3
Bromobutyl rubber, 2222           100     80     60
Natural rubber, SMR 20              0     20     40
Processing aid, SP 1068             4      4      4
Carbon black, N660                 60     60     60
Processing aid, Struktol 40 MS      7      7      7
Processing oil, TDAE                8      8      8
Stearic acid                        1      1      1
Zinc oxide                          1      1      1
Sulfur                           0.50   0.50   0.50
Accelerator, MBTS                1.25   1.25   1.25

Figure 1--inflation pressure retention--tire test results

% loss/month

BIIR/NR

100/0     1.48
80/20     2.05
60/40     2.75

H-rated tires
[R.sup.2.] = 0.9965

BIIR/NR

100/0     1.45
80/20     2.00
60/40     2.65

S-rated tires
[R.sup.2.] = 0.9977

Note: Table made from bar graph.

Figure 2--intracarcass pressure--tire test results

% loss/month

BIIR/NR

100/0      57.9
80/20      82.5
60/40     116.8

H-rated tires
[R.sup.2.] = 0.9965

BIIR/NR

100/0      55.1
80/20      75.1
60/40     113.9

S-rated tires
[R.sup.2.] = 0.9977

Note: Table made from bar graph.

Figure 3--tread separation tire roadwheel
(28.5 cm) durability test results

Hours to failure

Tire IPR
loss rates

1.48         797.2
2.05         668.0
2.75         610.6

H-rated tires
[R.sup.2.] = 0.953

Hours to failure

Tire IPR
loss rates

1.45         646.0
2.00         461.9
2.65         389.9

H-rated tires
[R.sup.2.] = 0.953

Note: Table made from bar graph.

Figure 4--FMVSS 139 endurance (to failure)
roadwheel (1.7 m) durability test results for
P205/60 HR15 tires

Modified FMVSS 139 endurance: H-rated tires
[R.sup.2] = 0.8827

Hours to failure

Inflation
pressure
retention

1.45        539.4
2.1         384.8
2.75        349.9

Note: Table made from bar graph.

Figure 5--FMVSS 139 high speed,
endurance, and low inflation (to failure)
roadwheel (1.7 m) durability test results
for P205/60 SR15 tires

Modified FMVSS 139 endurance: S-rated tires

                   Hours to failure

Inflation
pressure    High speed   Endurance    Low inflation
retention   to failure   to failure   to failure

1.45           86.4        97.0          18.1
2.00           57.2       100.7          24.2
2.65           84.5        90.1          26.7

Note: Table made from bar graph.

Figure 6--fixed oxygen contents for
P205/60 HR15 tire tested to failure on
the tread separation tester: top = raw
data, bottom = normalized data

Inflation
pressure
retention   Skim coat

1.48         3.76
2.05         3.77
2.75         4.03

Fixed oxygen: H-rated tires
[R.sup.2] = 00=.825

Normalized fixed oxygen skim: H-rated tires
[R.sup.2] = 0.99999

Inflation
pressure
retention   Skim coat

1.48         3.76
2.05         4.27
2.75         4.89

--Linear (skim coat)

Note: Table made from bar graph.

Figure 7--peel strengths and total cross-links
for oven-aged and roadwhell-tested P205/60 SR15
tiers for skim (left) and wedge (right) components

Peel strength: FMVSS 139LI

Inflation pressure retention

            Lb./in.

      1.45   2.00   2.65

Skim  22.8   19.5   22.6
Wedge 21.7   18.9   20.3

[GRAPHIC OMITTED]

Note: Table made from bar graph.

Figure 8--fixed oxygen contents for oven-aged
and roadwheel-tested P205/60
SR15 tires for skim coat (top) and
shoulder wedge (bottom) components

Fixed oxygen skim: S-rated tires after oven aging and
FMVSS 139LI testing - [R.sup.2] = 0.9994

Inflation pressure retention    %

1.45                           3.41
2.00                           3.53
2.75                           3.65

Note: Table made from bar graph

Fixed oxygen wedge: S-rated tires after oven aging and
FMVSS 139LI testing - [R.sup.2] = 0.999

Inflation pressure retention    %

1.45                           3.33
2.05                           3.46
2.75                           3.58

Note: Table made from bar graph

Figure 11 - results of shearography
crackling for the oven-aged and
roadwheel-tested P205/60 SR15 tires

Shearography: FMVSS 139LI of air-aged tires
(4 wks. @ 70[degrees]C) - [R.sup.2] = 0.9998

Inflation pressure retention   [mm.sup.2]

1.45                             642
2.0                              12,729
2.65                             25,344

Note: Table made from bar graph